The present invention is related to the field of optical position encoders.
Optical position encoders are often sold in an unassembled or “kit” form so as to be integrated into higher-level assemblies. In one common configuration, the optical encoder includes a scale, such as an elongated linear scale, that is to be placed on an item whose position is to be measured. The encoder also includes a sensor assembly that is to be mounted in a relatively stationary position immediately adjacent to the scale. The sensor assembly includes an optical source, such as a laser diode, and a photodetector. Light from the source is diffracted by a grating pattern on the scale and produces an interference pattern at the detector. Motion of the scale produces a corresponding apparent motion of the interference pattern which is detected by the detector. The detector output(s) are processed by electronic circuitry to convert the raw detector signals into a higher-level indication of position (such as a binary word) that can be utilized by the system.
For best operation of the encoder, it is necessary that the sensor assembly and the scale be accurately aligned. In a linear encoder, for example, it is desirable that the sensor be aligned with the scale in a Y-axis direction extending perpendicular to both an X-axis direction of relative motion and a Z-axis direction of separation of the scale and sensor assembly. In addition to this linear Y-axis alignment, it is also necessary to achieve a desired rotational or “theta-Z” alignment between the sensor assembly and the scale about the Z axis. Among other benefits, achieving the correct theta-Z alignment can improve signal strength and quality from the detector. Similar considerations apply in a rotary encoder.
It has been known to employ a so-called Lissajous display to ascertain the alignment between a scale and a sensor assembly in an optical encoder. The Lissajous display can be used in connection with encoders that implement quadrature-based sampling of the optical diffraction pattern. One or more detector output signals are used to derive a sine signal, and other detector signal(s) are used to derive a cosine signal. These signals are displayed together on orthogonal axes of an oscilloscope. As the Y and theta-Z alignment are changed, the roundness and size of the Lissajous pattern changes in a corresponding manner, and thus can be used to estimate when correct alignment has been achieved.